1. Field of the Invention
The present invention relates generally to magnetic levitation systems for moving objects, and more specifically, to a means for damping deleterious oscillations involving magnetic levitation train systems.
2. Description of Related Art
Halbach arrays, invented by Klaus Halbach in the 1980s for use in particle accelerators, represent a maximally efficient way to arrange permanent-magnet material when it is desired to produce a strong periodic magnetic field adjacent to the array. The beauty of the concept is that the effect of the cross-magnetized magnet bars in the array is to enhance the periodic magnetic field at the front face of the array, while canceling it on the back face of the array. Not only is the field enhanced, but analysis shows that in a long array the horizontal and vertical components are nearly purely sinusoidal in their spatial variation, with negligible higher spatial harmonics. If the Halbach array is then fabricated from high-field permanent-magnet material, such as NdFeB, peak fields near the front face of the array of order 1.0 Tesla are possible.
In the Inductrack maglev system, Halbach arrays are used, located below the train car. When in motion the magnetic field of these arrays then induces currents in a special “track” made up of close-packed shorted circuits. Analysis has shown that the combination of the three elements, Halbach arrays, NdFeB magnet material, and close-packed circuits in the track result in the possibility of achieving levitation forces in excess of 40 metric tons per square meter of levitating magnets, corresponding to magnet weights of only a few percent of the levitated weight.
Background information for maglev systems can be found in U.S. Pat. No. 5,722,326, titled “Magnetic Levitation System For Moving Objects”, U.S. Pat. No. 6,663,217 B2, titled “Inductrack Magnet Configuration”, and U.S. Pat. No. 6,664,880, titled “Inductrack Magnet Configuration”, all to Richard F. Post, all assigned to the assignee of the present invention, and all of which are incorporated herein by reference in its entirety.
However, such maglev systems as disclosed in the incorporated applications often produce deleterious resonant frequency oscillations of at least about 1 Hz in all principal modes of motion when in operation. These oscillations are describable as rolling, pitching, or yawing motions, or longitudinal oscillations superposed on their normal forward or backward velocity. These oscillations can be stimulated by imperfections in the “track,” by the drive system, or by inherent instability arising from some special characteristic of the levitation system itself. Typically the levitation system is not capable, of itself, to damp out these oscillations and other means is therefore required.
A general rule for achieving effective damping of driven oscillations (e.g., rolling, pitching, or yawing motions, or longitudinal oscillations superposed on their normal forward or backward velocity of a moving levitated system) is that the kinetic energy associated with the oscillations be dissipated in the stationary frame, that is, in the track and/or its supporting structure. While in some cases damping can be effected in the moving frame (i.e., in the frame of the moving object), this type of damping is not only characteristically less effective, but also may be the driving source for instabilities of the “dissipative” type, a type encountered in other contexts, such as in bearing-supported rotating systems.
Accordingly, a need exists for damping the induced oscillations at these slow frequencies in such maglev systems. The present invention is directed to such an invention.